I have a problem with the implementation of a linear-scaled
frustum-culling method (means: without quad-tree or something like that).
I have followed the implementation shown at http://www.flipcode.com/articles/article_frustumculling.shtml.
There is a paper mentioned on how to extract the frustum out of the
modelview and projection matrices
(http://www2.ravensoft.com/users/ggribb/plane%20extraction.pdf)
which I have also read and implemented.

The odd thing is, that the objects (trees) which I want to cull are
culled "to early" as if the far pane has wandered nearer towards the
camera-position. I could trace the error to the computation of the
far-clipping pane in my code. If I divide it by the scaling-factor
for the trees then the culling works fine.

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An excerpt from my source-code is as follows:
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void RenderGLScene()
{
// ...
glPushMatrix();

// scale the map
glScalef(20,20,20);
// translate the map
glTranslatef(-HEIGHTMAPSIZE/2,0,HEIGHTMAPSIZE/2);
// render the map
renderHeightMap();
// render the trees on it which are completely billboarded
renderTreeMap();